This invention relates to the bonding of a first element to a second element by means of soldering or brazing. More particularly, the present invention relates to a brazing or soldering alloy of relatively high melting point.
In general, the term "brazing" is employed with respect to the making of joints with a joining alloy of relatively high melting point while the term "soldering" has come to refer to the making of joints with a joining alloy of lower melting point. At times, the term "soldering" is used in a generic sense to include operations customarily referred to in the art as "brazing".
More particularly, the present invention relates to the bonding of input/output electrical connection pins and other components of a chip carrying substrate in an electronic system. The alloys of this invention are useful in electronic circuit interconnections, such as a chip carrying substrate and its pins, in a manner compatible with the continuous reheating of the chips.
Problems have developed in the use of bonding alloys in the bonding of pins or the like to electronic chip carrying substrates, such as multilayered ceramic substrates, which require reworking, i.e., removal and replacement of chips on the substrate. The bonding alloy used can begin to melt at the high reworking temperatures employed, say as high as 370.degree. C., with resultant movement or tilting of circuit connection pins which places them out of alignment, or causes general weakening of the pin-substrate bond. There is a need in the art for a new brazing or soldering material which remains as a strong joint at the high temperatures used for reworking chip carrying substrates of electronic systems. Thus, there is a need to provide alloys which can bond pins and the like to the substrate, and the substrate to its carrier, with the alloy having a melting point such that the joint comprising the alloy is unaffected by heating the overall structure to a temperature required for melting of the soldered connections retaining chips upon the substrate.
A bonding alloy of eutectic gold and tin in a ratio of 80 parts gold to 20 parts tin, which alloy melts at 280.degree. C., has been used to bond pins to the substrate in an electronic system. However, chip joining can be done at a temperature up to about 360.degree. C. This means that during chip joining or chip replacement, the braze is in the liquid state. Thus, the pins can easily be moved and in addition the liquid metal alloy can also react very rapidly with bonding surfaces, resulting in overall reliability problems. Obviously, braze alloy melting above the chip joining temperature would be more stable and eliminate the reliability problem.
A binary gold-indium alloy, 73 parts gold to 27 parts indium, melts at 456.degree. C. and has been used for brazing; however, because of extreme bittleness, the use of this alloy is limited.
U.S. Pat. No. 2,464,821 by Ludwick et al discloses a binary gold-indium brazing alloy in the proportion of 85 parts gold to 15 parts indium. U.S. Pat. No. 2,813,790 by Hack discloses a ternary brazing alloy of 60 parts gold to 30 parts copper to 5 parts indium.